This project focuses on the mechanism and function of immmune response (Ir) genes in the activation of thymus-derived (T)-lymphocytes using a secondary T-cell proliferation assay and the recently developed technology of T-lymphocyte cloning. With these techniques we have demonstrated that Ir gene complementation stems from the requirement for two separately encoded polypeptide chains to form a single Ia molecule involved in antigen-presentation. Furthermore, we have accumulated evidence that two separate sites exist on the antigen molecule, pigeon cytochrome c fragment 81-104, and that the recognition of only one of these sites appears to be under Ir gene control. These results support presentational models of Ir gene function. Finally, we have isolated clones of antigen-specific T lymphocytes and shown that some of them also possess alloreactivity. This result demonstrates conclusively that these two T cell repertoires are over-lapping sets.